Field of the Invention
The present invention relates to an electrically-driven compressor for vehicle, and particularly to control of an inverter that drives a motor of the compressor.
Description of the Background Art
An electrically-driven compressor mounted on a vehicle includes an inverter device for controlling a motor that drives the compressor. Such inverter device controls an output current by the high-frequency switching operation of switching elements and thereby controls the rotation speed of the motor. The inverter device outputs the current to coils of the motor by pulse width modulation (PWM) control.
In order to prevent a short circuit caused by the switching elements in upper and lower arms being simultaneously turned on, the dead time is provided to various drive signals for the switching elements used in the inverter device executing the PWM control.
During the dead time period, the switching elements in the upper arm and the switching elements in the lower arm are both turned off, and thus, an intended voltage is not applied to the motor. Therefore, an error occurs in a voltage output applied to the motor, which adversely affects the motor operation control.
Therefore, a method for correcting the voltage output during the dead time period as described in Japanese Patent Laying-Open No. 5-184157, WO2008/053538, Japanese Patent Laying-Open Nos. 2014-212606 and 2015-80294 is under study.
The error of the voltage output occurring during the dead time period depends on the direction of a motor current flowing during the dead time. Therefore, correction of the dead time in accordance with the motor current is needed.
On the other hand, reduction in size is required in the vehicle-mounted compressor, and thus, the motor of the compressor must also be rotated at high speed for the same work. If the motor is PWM controlled with the same control cycle as a control cycle at the time of low-speed rotation, the number of control cycles per rotation is reduced at the time of high-speed rotation, which leads to coarse control.
At the time of high-speed rotation, the error occurring during the dead time period has a great influence. Specifically, a voltage waveform in the PWM control has the large number of pulses per rotation at the time of low-speed rotation, and thus, recovery is possible during one rotation even when there is an error in one pulse. In contrast, the number of pulses per rotation is smaller at the time of high-speed rotation. Therefore, when there is an error in one pulse, the error has a great influence on the rotation control and distortion of rotation occurs, for example.
Therefore, when dead time correction is made as usual without exception, the error occurring during the dead time period may have a great influence on rotation.
Shortening the control cycle is also conceivable. However, due to efficiency of the hardware such as a CPU, there are limitations on shortening the control cycle.